1. Field of the Invention
The present invention relates to a diffraction/optical device and an optical system including the diffraction optical device. More particularly, the present invention is intended to realize a diffraction optical device capable of suppressing the occurrence of flare light due to diffracted light of unnecessary orders, and an optical system including the diffraction optical device.
2. Description of the Related Art
Hitherto, there is known a method for reducing chromatic aberration by combining plural kinds of glass materials with each other. Another advanced method for reducing chromatic aberration by providing a diffraction optical device 3, which develops a diffraction action, on a lens surface or in part of an optical system is disclosed in the literature of SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Patent Laid-Open No. 4-213421 and No. 6-324262, U.S. Pat. No. 5,044,706, etc. This method for reducing chromatic aberration is based on the physical phenomenon that a refracting surface and a diffracting surface in an optical system develop chromatic aberration in opposing directions for light of a certain reference wavelength.
Further, providing a diffraction optical device is greatly effective in reducing the aberration of an optical system because the diffraction optical device is able to function similarly to an aspherical lens by changing the grating pitch so that its diffractive power is partly changed.
While in a refraction optical system one ray of light remains as it is after being refracted, one ray of light is divided into plural rays of diffracted light of different orders in a diffraction optical system. In the case of employing a diffraction optical device in a lens system, therefore, the structure of a grating must be determined such that light in the wavelength range to be used is concentrated in one particular order (referred to also as the “design order” hereinafter). By concentrating diffracted light in the design order, diffracted light of other orders has a low intensity and can be regarded as being absent if the intensity is zero.
If rays of diffracted light of orders other than the design order are present, those light rays are focused in positions different from that in which the ray of diffracted light of the design order is focused, and hence generate flare light that is out of focus with respect to the design image plane. For this reason, in an optical system utilizing the diffraction effect, it is important to pay due consideration to the spectral distribution obtained with the diffraction efficiency for diffracted light of the design order and the behavior of diffracted light of orders other than the design order. Thus, to effectively utilize the color-aberration compensating effect of a diffraction optical device having the above-mentioned properties, it is required that the diffraction efficiency for diffracted light of the design order is sufficiently high over the entire wavelength range to be used, an diffracted light is substantially concentrated in the design order.
FIG. 7B shows a characteristic of the diffraction efficiency resulting when a diffraction optical device shown in FIG. 7A is formed on a certain surface in an optical system.
In the following description, the value of the diffraction efficiency is defined by the ratio of the amount of diffracted light of each order to the total amount of light passing the diffraction optical device. For brevity of explanation, however, light reflected by the boundary surface of a grating, etc., are not taken into consideration in calculating the value of the diffraction efficiency. In FIG. 7B, the horizontal axis represents wavelength and the vertical represents diffraction efficiency.
The diffraction optical device comprises a grating with a pitch (period) of 200 μm and a height of 1 μm. The grating is made of a material having a refractive index nd=1.513 and the Abbe's number vd=50.08. The grating has a glazed structure as shown in FIG. 7A. The graph of FIG. 7B indicates the diffraction efficiency when the incident angle is zero (0 degree). This diffraction optical device is designed such that the diffraction efficiency in the wavelength range to be used is maximized for diffracted light of 1-order (indicated by a solid line in FIG. 7B). In other words, the design order 1-order. FIG. 7B also represents the diffraction efficiency for light of orders around the design order (1-order ±one order, i.e., 0- and 2-order indicated respectively by a broken line and a one-dot-chain line in FIG. 7B).
As shown in FIG. 7B, the diffraction efficiency for light of the design order is maximized at a certain wavelength (design wavelength) and is gradually lowered as the wavelength departs away from the design wavelength. Corresponding to a lowering of the diffraction efficiency for light of the design 1-order, diffracted light of other orders (0- and 2-orders, etc.) occurs and gives rise to unwanted flare light.
Japanese Patent Laid-Open No. 9-127322 discloses an arrangement capable of suppressing a lowering of the diffraction efficiency at wavelengths other than the design wavelength. With this related art, high diffraction efficiency is realized over the entire visible range by selecting three kinds of materials and two different grating thickness in optimum combinations, and arranging a plurality of gratings in an adjacently superimposed relation with an equal pitch distribution.
Another arrangement capable of suppressing a lowering of the diffraction efficiency is disclosed in Japanese Patent Laid-Open No. 10-133149. Gratings are superimposed one above the other to have a two-layered sectional shape. High diffraction efficiency is realized over the entire visible range by optimizing the refractive indexes of materials of the two-layered gratings, the dispersion characteristics thereof, and the thickness of reach grating.
According to the techniques disclosed in the above-cited publications, a diffraction optical device is made of two or more kinds of materials having different dispersion characteristics to reduce phase shifts occurring at wavelengths other than the design wavelength when light passes the diffraction optical device. As a result, the dependency of diffraction efficiency of the diffraction optical device upon wavelengths is greatly suppressed.
By arranging the diffraction optical device in a refraction optical system, color aberration can be reduced to a large extent based on the physical phenomenon that the direction of dispersion of the diffraction optical device is opposed to that of a refraction optical device. It is also possible to compensate for other aberrations by utilizing the above-mentioned effect that the diffraction optical device is able to function similarly to an aspherical lens.
In the diffraction optical device of the related art, however, the grating has a large depth and the dependency of diffraction efficiency upon the incident angle of light upon the diffraction optical device is increased. This raises the problem that the diffraction efficiency is greatly reduced depending upon the layout of the diffraction optical device in the optical system.
Particularly, when an air layer is formed between two gratings made of materials different from each other as disclosed in Japanese Patent Laid-Open No. 11-223717, the flexibility in the selection of the grating materials is greater than that in the diffraction optical device disclosed in the above-cited Japanese Patent Laid-Open No. 10-133149, but the dependency of diffraction efficiency upon the incident angle of light is further increased.